Rotatable vessels are used in a variety of systems. Rotating vessels may be used in the chemical process industries and can include a rotatable cylinder or other shaped vessel that is supported on a rotatable shaft. These types of systems can include ball mills, blenders, dryers, filters, and reactors.
Some common shapes used for the rotatable vessel include a double cone shape, an offset cone shape, a V-shaped vessel, a vertical cylinder, and a horizontal cylinder. For each of these shapes, the vessel may be mounted on a shaft that defines the axis of rotation. The shaft is rotated by a drive mechanism, and rotation of the shaft will cause the vessel to rotate to perform the intended operation.
These vessels may be loaded and discharged with solids, liquids, or a combination of a solid or liquid. Loading and discharge may occur through various ports, hatches, valves, or other opening located on the periphery of the vessel. In addition to loading and discharging, additional operations may be performed via these openings, such as product sampling, inspection, and cleaning.
To perform these operations, the vessel may be positioned at a predetermined location that corresponds to the desired operation. For example, for loading the opening may be positioned to be at the top of the vessel. The positioning occurs via rotation of the vessel to the desired rotational position. For discharge, the opening may be positioned at the bottom of the vessel.
To perform these operations, such as loading, discharging, or the like, an auxiliary flange may be connected to the opening. The flange may be stationary or retractable, and can make the connection from above or below, depending on the operation. Thus, the opening may dock with the flange to perform the desired operation. For example, the opening may dock with the flange of a supply of product that is to be loaded into the vessel, or the opening may dock with the flange of a discharge hopper or conduit for receiving product out of the vessel.
Positioning of the vessel can be automated through the use of a variable frequency drive, which can slow the vessel down, and through the use of a brake, which can stop and hold the vessel in the desired position. However, when loaded with product, the vessels may have considerable weight, in some cases exceeding 50,000 pounds. When loaded with product, the process of slowing, positioning, and stopping the vessel can be difficult to automate to a desired position with precision.
To determine the position of the vessel, a positioning disc with magnetic properties or other sensors may be used. Alternatively, an encoder or counter may be mounted on the motor. A programmable logic controller or other electronic or pneumatic logic can be used to sense the position of the vessel, slow the speed of the vessel, and lock the vessel in position with the brake. With this type of control, it is possible to position the vessel within a few degrees of a desired location.
However, even when the brake is in a locked position and holding the rotor of the drive motor in place, there can be backlash or play in the gearing. The type of gear reduction, the size of the vessel, and the rotational speed can affect the amount of backlash and play. Rotating a vessel at a slower speed can require more reduction and therefore more gears, and an increase in gears can increase the amount of backlash and play.
Thus, while the positioning system can be somewhat accurate in stopping the vessel near the desired and correct positon, the backlash allows the vessel to move relative to the desired position, and the final position of the vessel and accuracy of the final position can be difficult to achieve.
In some cases, a generally correct positioning within a few degrees may be adequate. However, many rotatable vessel processes require inert atmospheres, or may involve highly toxic or biologically active chemicals. In such instances, a tight and reliable seal with the docking flange may be required. In the case of toxic and biologically active chemicals, the need for a human operator to make or break the connection between the vessel and the docking flange could be too hazardous. In the case of automated docking via a robotic arm, automated extension flanges, or other mechanisms, the final position of the vessel may need to be more accurate.
Accordingly, improvements can be made in the positioning of rotatable vessels.